Radiation dosimeter using gas of greater electron affinity and molecular moss than air

ABSTRACT

In a radiation dosimeter, wherein a sealed container encloses an ionizable gas and dielectric members for separating a triboelectric charge, the improvement wherein the ionizable gas comprises a material having a greater mass and greater electron affinity than air. Specific materials include sulfur hexafluoride and fluorinated hydrocarbons.

United States Patent Richter [54] RADIATION DOSIMETER USING GAS OF GREATER ELECTRON AFFINITY AND MOLECULAR MOSS THAN AIR [72] Inventor: Henry L. Richter, Pasadena, Calif.

[73] Assignee: Dr. Henry L. Richter Corporation [22] Filed: Sept. 15, I969 [2i] App]. No.: 858,031

7 [52] US. Cl. ..250/83.3 PD

Field ofSearch ..250/83, 83.3, 83.3 PD; 313/93 56] References Cited I UNITED STATES PATENTS 2,700, I09 1/1955 Argabrite .;.250/83.3

15] 3,665,197 [451 May 23, 1972 2,731,568 1/1956 Failla "250/833 2,776,390 l/l957 Anton 3,075,081 1/1963 Landsverk et al. ..'.2$0/833 Primary Examiner-James W. Lawrence Assistant Examiner-Davis L. Willis Auomey-Nilsson, Robbins, Willis and Berliner ABSTRACT In a radiation dosimeter, wherein a sealed container encloses an ionizable gas and dielectric members for separating a triboelectric charge, the improvement wherein the ionizablegas comprises a material having a greater mass and greater electron affinity than air. Specific materials include sulfur hexafluoride and fluorinated hydrocarbons.

7 Claim, 1 Drawing Figure Patented May 23, 1972 3,665,197

INVENTOR.

HE/V/QY L. RICHTER.

'ul tg/J BACKGROUND OF THE INVENTION 1. Field of the Invention The field of art to which the invention pertains includes the field of detection of invisible radiant energy.

2 Description of the Prior Art A variety of devices have been proposed for detecting and measuring gas ionizing radiation. Among these devices are those relying on the creation of a triboelectric charge to suspend a plurality of dielectric elements within an enclosed chamber containing an ionizable gas, as illustrated in U.S. Pat. Nos. 2,700,109, 2,731,568, 3,093,737 and 3,112,400. These devices are of relatively simple construction and operation and areeconomical of manufacture, and are thus suitable for mass distribution and use by unskilled personnel.

In use, such devices are charged by being shaken or otherwise agitated in a manner to distribute the plurality of dielectric elements, generally in the form of small pellets, along a dielectric surface within the device. Such agitation developes a triboelectric or static charge on the dielectric surface and a static charge of opposite polarity on the pellets. This causes the pellets to repel one another and to be attracted to and adhere suspended in a random pattern along the'dielectric sur- DETAILED DESCRIPTION Referring to the drawing, there is shown a dosimeter 10 exemplary of devices utilizing the improvement. The dosimeter 10 comprises a sealed container 12 enclosing the tubular member 14 which defines an ion chamber 16 therein which, in turn, encloses a plurality of dielectric pellets 18. The pellets 18 are comprised of any dielectric material, such as acrylic, having the requisite property of acquiring a triboelectric or electrostatic ch'argewhen agitated. The tubular member 14 is comprised of a different dielectric material, such as polystyrene. The ion chamber 16 is provided with a roof 20 having an aperture 22 therethrough to effect communication between the ion chamber 16 and the interior 24 of the container 12. The walls of the container 12 can be formed of any transparent material such as glass or transparent plastic; A disk of cobalt chloride or its equivalent can be provided on the floor of the container to change color when the interior 24 of the container is contaminated with moisture, as well known to the art.

In the prior art, vent orifices are provided to permit dehydration, of the contents of the container and then hermetiface. However, should external ionizing radiation be present,

these electrostatic charges willbe reduced in proportion to the quantity of the ionizing radiation which has penetrated the container. When any one or more of the pellets has its charge reduced to null by such ionizing radiation, such pellet or pellets will drop to the bottom of the device.

The sensitivity of such devices depends on several factors including the shape of the components, the size of the pellets, the thickness of the container walls, etc. Generally a device constructed in accordance with U.S. Pat. No. 3,112,400 is sensitive to about 750 milliroengtens, that is, after exposure to about 750 milliroengtens the pellets will begin to drop to the bottom of the device. While such sensitivity is generally satisfactory for most purposes, it would be desirable to increase the sensitivity without detracting from the physical characteristics of the device so as to allow even earlier warning of radiation hazard and to readily detect low level radiation leakage, e.g. from a color television set.

SUMMARY OF THE INVENTION The present invention provides a means for increasing the sensitivity of devices as above-described. The foregoing devices typically utilize air as the ionizable gas. I have discovered that by replacing part or all of the air with certain other gaseous ionizable material, the sensitivity of the devices can be dramatically increased. Specifically, l utilize gaseous material having a greater modecular mass and greater electron affinity than air. For example, by replacing all of the air with chlorotrifluoromethane (Freon 13), sensitivity of the device described in U.S. Pat. No. 3,] 12,400 is increased to 50 milliroengtens, that is, the device discharges when exposed to 50 milliroegtens of radiation. Another example of a gaseous material-having a greater affinity for electron than air and which effects a comparable increase in sensitivity is sulfur hexafluoride.

The foregoing utilization of Freons and sulfur hexafluoride are quite unusual in view of the well known dielectric properties of these material and the face that these very materials have been favored for use as insulating media for electrostatic generators, where ionizing radiation is not relevent. Accordingly, my discovery of their effectiveness for an opposite purpose, to effectelectrostatic discharge in the present dosimeters, is significantly unobvious.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a vertical sectional view of an exemplary dosimeter utilizing the improvement of this invention.

cally sealed, as at 30 and 32. However, in the present invention, these orifices are utilized in order to introduce the gaseous material of this invention into the ion chambers 16 and interior 24 of the device. In accordance with this invention, a gas such as chlorotrifluoromethane (sold under the trade name Freon 13') is introduced into the ion chamber 16 via the orifice 30 thereto and exhausted via the orifice 32 in the container wall 12 until sufficient volume of such gaseous material has passed through the device to exhaust the air therefrom and completely fill the ion chamber 16 and interior 24 of the container. The orifices 30 and 32 are then hermetically sealed.

As noted above, in experiments utilizing a dosimeter similar to the foregoing, but filled with air, exposure to about 750 milliroegtens was required before any of the pellets l8 dropped to the bottom of the ion chamber. However, when the air was substituted with chlorotrifluoromethane, as. above, pellets 18 fell to the bottom of the ion chamber upon being exposed to finity for electrons, the free electron tends to almost immediately recombine with the positive ions or with a previously formed ion adjacentthereto. Therefore the lifetime of the electron is relatively short and as a result thereof the opportunity for the electron and/or the ion to travel towards and be captured by the electrostatic field in the dosimeter is relatively small. Discharge of the apparatus is thus prolonged since many more ions and electrons must be formed to discharge the apparatus.

However, by utilization of a gaseous material having a high electron afiinity, when the gaseous material is ionized by radiation, removed electrons have a greater tendency to attach themselves to adjacent molecules of the gas, enhanced by the greater electron affinity of the gaseous material. Further, since the mass of the gaseous molecules utilized herein is much greater than the average mass of the molecules of air normally utilized in prior devices, the lifetime of the free electron is greatly increased and the triboelectric charge between the pellets 18 and chamber wall 14 has a greater opportunity to attract the molecule of gas having the free electron associated therewith as well as the positive ion.

Theelectron affinity of a molecule (M) can be defined by the exothermicity of the reaction M e M in the gas phase. Values for such reactions for a variety of materials, as well as calculated and measured electron affinities can be found in the literature. See, for example: L.M. Branscomb, in Atomic and Molecular Processes, Ed. D.R. Bates, Academic Press, New York, (1962); I-I.O. Pritchard, Chemica] Reviews, 52, 529 (1953); and F.M. Page, Technical als which can be substituted for the chlorotrifluoromethane include: sulfur hexafluoride; sulfur dioxide; bromofluoromethane; chlrordifluoromethane; dichloroditluoromethane; trichlorofluoromethane; bromotrifluoromethane; l,2-dichloro-l ,1,2,2-

tetrafluoroethane, 1,1,2 trichloro 1,2,2 trifluoroethane; and 1,2 dibromo l,l,2,2 tetrafluoroethane.

It is not necessary to replace all of the air with the substitute gaseous material to achieve increased sensitivity since even small amounts of the substituted material will enhance the sensitivity. As a practical matter, anywhere from about 5 to 100 percent of the air can be substituted with one or any combination of the foregoing gaseous materials to enhance the sensitivity of the device. Responsiveness of the device to low level X-rays, e.g., from a color television set, can be enhanced by thinning the outer walls of the chamber until a desired level of detection is obtained.

What is claimed is:

1. In a device for visually indicating the presence of ionizing radiation including a sealed container, dielectric members within said container for separating triboelectric charge, and an ionizable gas within said container, the improvement wherein said ionizable gas comprises substantial amounts of material in gaseous form having a greater molecular mass and greater electron affinity than air, said material containing at least one fluorine atom, as a part only, in its molecular structure.

2. The improvement of claim 1 wherein said material is sulfur hexafluoride.

3. The improvement of claim 1 wherein said material is a fiuorinated hydrocarbon. I

4. The improvement of claim 1 wherein said material is chlorotrifluoromethane.

5. The improvement of claim 1 wherein about 5 to percent of ionizable gas within said container is comprised of said gaseous material.

6. The improvement of claim 5 wherein said gaseous material is chlorotrifluoromethane.

7. The improvement of claim 5 wherein said gaseous material is sulfur hexafluoride. 

2. The improvement of claim 1 wherein said material is sulfur hexafluoride.
 3. The improvement of claim 1 wherein said material is a fluorinated hydrocarbon.
 4. The improvement of claim 1 wherein said material is chlorotrifluoromethane.
 5. The improvement of claim 1 wherein about 5 to 100 percent of ionizable gas within said container is comprised of said gaseous material.
 6. The improvement of claim 5 wherein said gaseous material is chlorotrifluoromethane.
 7. The improvement of claim 5 wherein said gaseous material is sulfur hexafluoride. 